If you want to specify the concentration of a subset of the partaking species, this can be done by using the Concentration node instead.

For the Electroanalysis interface, this node is available when you select the Convection check box on the physics interface Settings window.

For the concentration of each species c0,c (SI unit: mol/m3), enter a value or expression.

The option Concentration constraint constrains the concentration values on the boundary by the use of pointwise constraints.

The other option, Flux (Danckwerts) can be used when the concentration at the boundary is not known, or when it varies in a non-trivial manner. This may, for example, be useful when reactions with high reaction rates occur in the vicinity of the inlet. In this case the concentration far upstream of the boundary is instead prescribed. The Flux (Danckwerts) condition prescribes the total flux defined by the upstream concentration and the fluid velocity at the boundary.

To display this section, click the Show button () and select Advanced Physics Options. You can find details about the different constraint settings in the section Constraint Reaction Terms in the COMSOL Multiphysics Reference Manual.

See the theory chapter in the section Danckwerts Inflow Boundary Condition.